The invention relates to a gas inlet for an ion source. The gas inlet should introduce the molecules (or atoms) to be ionized into the ion source in such a way that the highest possible ionization efficiency is obtained (that is, that a high sensitivity in the ionization step can be achieved).
It has so far been common practice to introduce the gas to be analyzed into the ion source of the mass spectrometer in an effusive manner. To that end, a supply line (for example, the end of a gas chromatographic capillary) leads to the ion source to which may be of a closed (as for example in many C1xe2x88x92 or xe2x88x92E1 ion sources for quadrupole- or sector field mass spectrometers) an open design (for example, many ion sources for travel time mass spectrometers (TOF-mass spectrometers)). In the case of ion sources of closed design, an area of the ion source is flooded by the admitted gas that is the admitted atoms or molecules partially collide with the ion source wall before they can be ionized and detected in the mass spectrometer. The open design of many ion sources for TOF mass spectrometers favors the use of atom- or molecule beam techniques. In that case, a relatively focussed gas beam is directed through the ion source, which gas beam has, in the ideal case, only very little interaction with the building components of the ion source.
For the travel time mass spectrometry effusive molecular beams [2] as well as skimmed [1] and unskimmed [3, 4] supersonic molecular beams are used (in each case, pulsed or continuous (cw)).
Supersonic molecular beam inlet systems permit a cooling of the gas to be analyzed in a vacuum by an adiabatic expansion. It is however a disadvantage that, in conventional systems, the expansion must take place at a relatively large distance from the location of ionization. Since the density of the expanding gas beam (and consequently the ion yield for a given ionization volume) drops exponentially with the distance from the expansion nozzle the achievable sensitivity is limited.
Effusive molecular beam inlet systems do not permit a cooling of the sample. However, gas inlet systems for effusive molecular beams can be so designed, that the gas is discharged directly to the ionization location by way of a metallic needle which extends to the center of the ion source. In that case, a certain electric potential is applied to the needle in order not to disturb the withdrawal fields in the ion source. The needle has to be heated to relatively high temperatures in order to prevent the condensation in the needle of the molecules of low volatility, which are to be analyzed. It is to be taken into consideration in this connection that the coldest point should not be at the needle tip. The required heating of the needle is problematic since the needle needs to be electrically insulated with respect to the rest of the structure (for example, by way of a transition part of ceramic material). Electric insulators are generally also thermal insulators and therefore permit only a very low heat flow from for example the heated supply line to the needle. Heating by electric heating elements or infrared radiation is also difficult since the needle extends between the withdrawal plates of the ion source.
The selectivity of the resonance ionization with lasers (REMPI) depends on the inlet system used (because of the different cooling properties). Besides the effusive molecular beam inlet system (EMB), which can be used among others for the detection of complete classes of substances, it is possible, by the use of a supersonic molecular beam inlet system (jet), to ionize in a highly selective manner and partially even in an isomer selective manner. With the commonly used supersonic nozzles, which were developed for spectroscopic experimentation the utilization of the sample amount (that is, the achievable measuring sensitivity) is not a limiting factor. Furthermore, the existing systems are not designed so as to avoid memory effects. For the use of REMPI-TOFMS spectrometers for analytical applications, the development of an improved jet or beam inlet system is necessary. It has to be taken into consideration however that the valves must consist of inert materials in order to prevent memory effects or chemical decomposition (catalysis) of the sample molecules. Furthermore, the inlet valves should not include any dead volumes. Also, the valves must be able to be heated to more than 200xc2x0 C. so that also compounds with low volatility of the mass range  greater than 250 amu are accessible. Further, as little as possible sensitivity should be lost by the jet arrangement as compared to effusive inlet techniques. This can be achieved mainly by a more effective utilization of the introduced samples in comparison with conventional jet arrangements.
This increase is achieved for example in that each laser pulse reaches the largest possible part of the sample. Under ideal conditions, the sample would be introduced in a pulsed form with each laser pulse so that no sample material is lost between the laser pulses. Furthermore, the injected sample beam should have a spatial extension corresponding to the laser beam. In this way, the complete sample would be used for the analysis without any losses. Then also relatively small sample amounts would produce an adequate signal at the detector. Since the withdrawal volume is predetermined by the dimensions of the laser beam (a widening of the laser beam would reduce the REMPI effective cross-section which scales for example with a two photon ionization with the square of laser intensity) it must be attempted to optimize the spatial as well as the time overlap of the molecular beam and the laser beam. Boesl and Zimmerman et al. [5] present for example a heatable jet valve for analytical applications, for example for the gas chromatography-jet-REMPI-coupling with minimized dead volume. For applications in the area of the ultra-trace analysis or the on-line analysis with REMPI-TOFMS, a further development with respect to the sample utilization (sensitivity), inertness (for example, avoiding metal-sample contact) and heatability (avoiding memory effects) is advisable. Pepich et al. presented a GC supersonic molecular beam-coupling for the laser-induced fluorescence spectroscopy, wherein, with the pulsed admission of the gas, an increase of the duty cycle was achieved in comparison with the effusive admission [6]. In order not to interrupt the GC flow by the pulsed inlet, Pepich has proposed to introduce the sample in an effusive manner into a pre-chamber into which the pulsed carrier gas is injected. In the process, the carrier gas compresses the analysis gas in the pre-chamber and pushes it, like a piston, downwardly through a small opening into the optical chamber where the fluorescence stimulation takes place. As a result of the pulsed compression and injection of the analysis gas into the optical chamber a larger amount of sample molecules can be involved in the subsequent laser excitation. The valve opening and the triggering of the laser must be so synchronized that the laser beam actually hits the area of the compressed analytes in the gas pulse. The arrangement makes also a repetitive, timely limited ( less than 10 xcexcs), compression of the sample possible without detrimentally affecting the GC-flow. The arrangement of Pepich et al., however, does not permit cooling of the sample gas (this can be achieved only by the installation of mixing structures such as glass wood for example, which detrimentally affects or even destroys the compression characteristics).
It is the object of the present invention to provide a gas inlet for an ion source in such a way that the expansion location of the gas beam can be directly in the ion source of a mass spectrometer in order to achieve a high sensitivity and, with the lowest possible gas loading of the vacuum, the highest possible sample concentration at the ionization location of the ion source of the mass spectrometer.
In a gas inlet structure for an ion source, including a capillary for the admission of a sample gas, which capillary is disposed in a guide tube for discharging a sample gas into the guide tube, the guide tube has an open end disposed in the ion source. The guide tube includes a valve for the pulsed admission of a carrier gas to the guide tube. The guide tube, the valve and the capillary are supported in a sealed support housing from which the guide tube with the capillary disposed therein projects into the ion source for supplying thereto the sample gas in a pulsed manner.
In comparison with the state of the art, the arrangement has the following advantages:
The supersonic molecular beam expansion can be placed directly into the ion source. In this way, in principle, the highest possible density of the gas beam at the ionization location is achieved. Furthermore, the arrangement permits the compression of the analyte gas in the gas jet pulse, which results in a further increased sensitivity. Particular advantages of the gas admission reside in the fact, that the sample is adiabaticly cooled, the capillary can be heated easily up to its lower end and the sample can be admitted in a pulsed manner.
The arrangement can be such that the sample molecules come in contact only with inert materials.
The injection of the gas should be possible either in a pulsed or in a continuous manner. Furthermore, the analyte gas pulses should be compressed by a driver gas pressure pulse in order to increase the detection sensitivity. By appropriate adjustment of suitable parameters, the gas can be cooled by an adiabatic expansion into the vacuum of the mass spectrometer (supersonic molecule beam or jet). The cooling of the injected gas is advantageous. The lower internal energy of cooled molecules results often in a lower degree of fragmentation in the mass spectrum. Particularly advantageous is the cooling for the application of the resonance ionization by lasers (REMPI). With the use of a so-called supersonic molecule beam inlet system (jet) for the cooling of the gas beam, it is possible to ionize with REMPI in a highly selective manner (partially even isomer-selectively). Since the gas is cooled by expansion, the sample gas admission line, the valve and the expansion nozzle can be heated without a substantial reduction in the cooling properties. This is important for analytical applications. Without sufficient heating, sample components can condense in the admission line or in the gas inlet. Important applications for the invention are the in-coupling of a chromatographic eluent or of a continuous sample gas flow from an on-line sample in a supersonic molecule beam. The inlet system described herein permits the location of the expansion into the ion source of the mass spectrometer. In this way, the ions can be generated directly below the expansion nozzle, which is very advantageous for the achievable detection sensitivity.
The invention will be described below on the basis of the accompanying drawings.